Two of Britain's major high street retailers launched inquiries last night into allegations that factory workers who make their clothes in India are being paid as little as 13p per hour for a 48-hour week, wages so low the workers claim they sometimes have to rely on government food parcels.

Primark, the UK's second biggest clothing retailer, and the Mothercare, the mother and baby shop, were responding to a Guardian investigation into the pay and conditions of workers in Bangalore, India, who supply several high-profile UK and US fashion brands.

The investigation, which follows our report in July in which Primark, Asda and Tesco were accused of breaching international labour standards in Bangladesh, has uncovered a catalogue of allegations of Dickensian pay and conditions in factories owned by exporters who supply clothes to the UK. India's largest ready-made clothing exporter, Gokaldas Export, which supplies brands including Marks & Spencer, Mothercare and H&M, confirmed that wages paid to garment workers were as low as £1.13 for a nine-hour day. This fails to meet their basic needs, according to factory workers and Indian unions and so falls below the minimum international labour standards promised by the Ethical Trading Initiative (ETI), a code of conduct which sets out basic rights for employees across the supply chain. Marks & Spencer is a member of the ETI, as are Mothercare, Gap and Primark.

Garment workers for factories owned by exporters who supply to Gap, Matalan and Primark, told the Guardian they were paid similar wages and regularly forced to work overtime of between six and 18 hours per week. The ETI code states that workers shall not regularly be required to work more than 48 hours per week, that overtime should be voluntary and that it should not exceed 12 hours per week.

Texport Overseas, which supplies Gap and Matalan, denied that workers were forced to do overtime.

Employees of factories owned by exporters who supply Gap and Matalan claimed they were often made to work extra hours without pay to meet unattainable production targets. They claimed the mostly female workforce was harassed and bullied by male production managers and supervisors for not achieving targets and that they were refused time off when ill. One worker claimed that security guards patrolled the toilets, harassing the women inside to get back to work, while the unions said public address systems were used to publicly humiliate and harass workers. Texport Overseas denied there was pressure to meet targets. It said female guards in the toilets were present to ensure "proper security", while the address system was to "coordinate production" and ensure health and safety.

One worker, a tailor who makes clothes for Gap, told the Guardian she was dismissed after being off work for more than 15 days due to illness. Texport Overseas denies this.

Another tailor who makes clothes for H&M, said that when she could not achieve her production targets, the clothes were thrown in her face. She said up to 15 workers a day collapsed and had to be given medical attention. Workers and unions claim the conditions in the factories led to two tragic incidents this year.

In February, a young woman hanged herself in the toilets of one factory, Triangle Apparels, owned by Gokaldas Exports. A report by a number of Indian NGOS alleges that she was verbally sexually harassed and repeatedly refused permission for leave on the day she died.

Jagadamba, 43, the mother of the dead woman, who now looks after her daughter's two sons, Sarti, eight, and Surya, four, told the Guardian that her daughter had only worked at the factory for 20 days, but had been very unhappy there. "She told me, 'They are always shouting at me because I can't meet the targets'."

Gokaldas denies that she was verbally sexually harassed. Triangle is not one of the factories supplying Marks & Spencer or Mothercare.

A month later, a nine-month pregnant woman from Shalina Creations, a factory supplying Gap, went into labour at work and subsequently lost her baby. Rathnamma, 27, a mother of two, claimed that she was refused immediate leave on March 29 this year, after going into labour. When she asked to go home, the production manager made her fill in forms that took an hour and a half, she said. "I was in such pain, I could hardly stand up."

When she finally made it outside the factory gates, she collapsed, she said, and gave birth to the baby in the street. A passerby helped her into an auto ricksaw, but when she got home, she discovered the baby was dead. Rathnamma, who has returned to work after being given paid leave for three months, said: "I feel angry. They gave me money, but nothing will bring the baby back. But I need the job. If I have no job, I have no food."

Gap representatives in the US did not dispute her allegations. However, a Gap representative in India denied that she was refused immediate leave, said that she gave birth in a rickshaw, and not on the street, and claimed the baby died when it slipped from her grasp.

KP Gopinath, the director of Cividep, an Indian workers' rights group, said: "When we speak to the workers, they tell us all they want is to be treated like human beings. They need a living wage to live in dignity, to get running water, to get a better education for their children."

He said a report by the Garment and Textile Workers union estimated a living wage in Bangalore to be at least £2.50 per day.

John Hilary from War on Want, said: "Exploitation of workers in developing countries such as India is standard practice for British retailers right across the spectrum. This just underlines the urgent need for Gordon Brown to step in now and stop these abuses once and for all."

A spokesman for Primark said that it took the allegations "very seriously".

He added that it had initiated immediate audits of all supplier premises and would re-audit to ensure compliance if necessary.

Mothercare also said it took the allegations seriously and would re-audit its two factories in India.

H&M said the harassment and forced overtime alleged was "unacceptable" and it would forward the complaints to its suppliers. It said it required suppliers to pay a legal minimum wage.

A statement from Gap said it regularly monitored suppliers, and had recently experienced a "number of compliance violations at factories in the Indian subcontinent region", and would work to resolve the issues.

A spokesman for Marks & Spencer said the two Gokaldas factories it used paid the legal minimum wage in India, as "there is no legal or industry-agreed" definition of a living wage. It added that it was working to "better understand this complex area".

Matalan said its suppliers paid above the minimum wage for Bangalore. Two compliance audits carried out on Texport Overseas in the past two years had found no record of forced overtime.